Governments, power utilities and researchers have recognized the need for a viable system to continuously detect and monitor mercury emissions in stack emissions and process gases. A number of proposed solutions exist in the prior art.
For example, some known conventional direct wet chemical analyzers use wet chemical reagents to condition gas samples for subsequent analysis by atomic absorption (AA) detectors, for example. Stannous chloride, sodium borohydride, or other chemical reductants may be used to convert the different mercury species to elemental form. However, these analyzers may suffer problems with sensitivity since the AA detectors generally have detection limits in the 1 μg/m3 range and cannot quantitate values of less than several μg/m3. While these analyzers may be useful in monitoring the emissions of waste incinerators for example, they are less useful in monitoring mercury in power plant emissions where greater sensitivity is required.
Another known wet chemical system uses a wet chemical front end, followed by gold preconcentration and detection using atomic fluorescence. The system samples full strength stack gas and splits the sample into a first path that uses an alkaline-based stannous chloride solution to convert all mercury forms into elemental mercury, and a second path that uses a tris-buffer or potassium chloride (KCl) solution to scrub out ionic mercury while passing elemental mercury. This system, however, requires the complex preparation of two different, low mercury reagents on a continuing basis, and is subject to high maintenance when used for extended periods of time. Use of liquid chemical agents may also produce toxic waste.
As a further example, conventional thermal conversion analyzers use thermal pyrolysis units to decompose the ionic mercury in gas samples into elemental form for subsequent analysis by AA detectors, for example, to determine a measure of the total mercury in the samples. Some known analyzers of this type utilize a stainless steel thermal pyrolysis unit coupled to a gold adsorption cartridge; mercury is adsorbed onto the cartridge during sampling and is thermally desorbed during an analysis phase. However, these analyzers typically suffer from recombination problems in the presence of hydrogen chloride (HCl) or chlorine (Cl2). In particular, the very poor transport characteristics of mercury chloride (HgCl2) means that this component of sample gas will not reach the gold cartridge in a timely manner, resulting in erroneous readings and memory effects. Even where direct AA analyzers are used, HgCl2 may not be detected at all within the AA cell since it does not absorb efficiently at the primary mercury adsorption line.
Some other known thermal conversion analyzers utilize carbon-based pyrolysis units. The problem with these pyrolysis units is that impure substances that prevent the reduction of the mercury or its release from the carbon, or which reoxidize already-reduced mercury, may accumulate in the carbon. In fact, conventional materials such as carbon, as well as quartz chips, stainless steel, alumina, and molecular sieve materials may produce excessive recombination after a period of continuous running, even where the concentration of stack gas components may have been greatly reduced through prior dilution.
Some attempts have been made to utilize solid sorbents such as calcium carbonate, sodalime, and calcium oxide to remove acid gases. However, these have not been applied commercially to a large extent, due to their very short lifetimes and their tendency to affect the accuracy of mercury readings towards the end of their lifetimes. The characteristics of solid sorbents may also change whenever the sample matrix changes.
The reliability of some other proposed remedies in preventing the formation of oxidized forms of mercury, such as the injection of hydrogen gas into a stack matrix after thermal dissociation, may also be questionable. The injection of hydrogen favors the creation of HCl, which is a powerful compound for causing recombination of elemental mercury into molecular species.